Reappraisal of mast cell leukemia based on a single institution review of 16 cases: Mast cell morphology determines clinical outcome
Authors: Animesh Pardanani, Ayalew Tefferi, Aref Al‐Kali, Mrinal Patnaik, William J. Hogan, Kebede Begna, Michelle A. Elliott, Nandita Khera, Jeanne M. Palmer, Naseema Gangat, Attilio Orazi, Katalin Kelemen, Kaaren K. Reichard, Dong Chen
Journal: American Journal of Hematology (2023)

Cytologic abnormalities of atypical mast cells in mastocytosis. The mature mast cells have oval-shaped nuclei, cytoplasmic hypogranulation and spindle-shaped cytology. or well-differentiated displaying a round nucleus with condensed chromatin, and abundant dense cytoplasmic granulations. Immature mast cells include promastocytes and metachromatic blast-like forms. Mastocytosis refers to the neoplastic proliferation of mast cells (MC), histologically apparent in the bone marrow (BM), skin, gastrointestinal system, liver, spleen, or any other organ.1 The first edition of the International Consensus Classification (ICC-2022) of myeloid neoplasms and acute leukemias considers mastocytosis as a distinct clinicopathologic entity, separate from myeloproliferative neoplasms (MPN) and myelodysplastic syndromes (MDS).2 The diagnosis of mastocytosis requires the presence of multifocal dense infiltrates of tryptase- and/or CD117+ MCs (≥15 MCs in each aggregate) detected in sections of BM and/or any of the aforementioned organs/tissues (major criterion) or the presence of at least three of four minor criteria: (i) >25% of MCs showing spindle-shaped or atypical/immature cytology, (ii) MC expression of CD25, CD2, and/or CD30, (iii) presence of activating KIT mutations, including KIT D816V, and (iv) persistent elevation of serum tryptase >20 ng/mL2, 3; the latter criterion does not apply in the presence of another myeloid neoplasm. Of note, diagnosis by ICC criteria requires absence of tyrosine kinase gene fusions, which are associated with myeloid/lymphoid neoplasm with eosinophilia (M/LN-Eo).3 ICC-2022 organizes mastocytosis into three categories: (i) cutaneous only (CM), (ii) systemic (SM), with or without skin involvement, and (iii) mast cell sarcoma2, 3; SM is, in turn, subtyped into five variants, based partly on the presence or absence of B (MC burden) and C (MC-associated organopathy) findings: indolent (ISM), smoldering (SSM), aggressive (ASM), SM with an associated myeloid neoplasm (SM-AMN), and mast cell leukemia (MCL)2; mast cell sarcoma is an extremely rare and deadly type of mast cell malignancy that is distinguished from MCL.4 To render a diagnosis of MCL, the revised 4th edition of the World Health Organization (WHO) classification system required the presence of ≥20% MCs in BM aspirate smear and mentions their usual morphology as being immature and atypical and as being round rather than spindle-shaped.5 The revised WHO classification also considered both leukemic (circulating MC ≥10%) and aleukemic variants of MCL.5 ICC-2022 was authored by the same group of pathologists and clinicians who oversaw the revised 4th edition of WHO and their revised diagnostic criteria for MCL retained the ≥20% cutoff for MCs in a BM aspirate smear but requires the morphological characterization to be atypical/immature6; in case of a dry tap aspirate, a BM biopsy showing diffuse infiltration by atypical/immature MCs was regarded as sufficient for MCL diagnosis.2 ICC-2022 acknowledges the occurrence of circulating MCs that might accompany MCL but does not endorse subcategorization into leukemic versus aleukemic MCL, and has also eliminated the use of the term “chronic MCL,” which refers to the mature morphology seen in some cases of MCL.7 The upcoming 5th edition of WHO classification, authored by a different set of investigators, also uses the 20% cutoff level for BM aspirate content of MCs but does not identify MC morphology as being integral to the diagnostic criteria and retains the poorly understood chronic MCL,7 as a distinct variant separated from the more common “acute” MCL, by the absence in the chronic form of “c” findings, as well as the distinction between leukemic and aleukemic MCL variants.8 The main objective of the current study was to examine the phenotypic and prognostic relevance of MC cytology in patients with MCL defined simply by the presence of ≥20% MCs in BM aspirate. The Mayo Clinic hematopathology and clinical databases were interrogated to identify MCL cases. Patients were retrospectively recruited after confirming research authorization and institutional review board approval. Diagnosis of MCL followed the revised 4th edition WHO classification criteria,5 in order to capture all cases with ≥20% MCs in BM aspirate smear, regardless of MC cytomorphology. Clinical and laboratory data were collected at the time of MCL diagnosis. Bone marrow slides were reviewed by 3 Mayo Clinic hematopathologists (KKR, KK, and DC), with each slide set being independently reviewed by at least 2 pathologists, using prespecified criteria. In addition, digitized images of the selected slides were reviewed by an expert external hematopathologist (AO). MC were cytomorphologically classified as being “mature” or “immature” (promastocytes, metachromatic blast-like forms, multinucleated or highly pleomorphic MCs, as previously described).6, 9 Patient follow-up was updated in July 2022; data were censored at last follow-up or at the time of allogeneic hematopoietic stem cell transplant (AHSCT). Conventional statistical methods were applied using JMP Pro 16.0.0 software (SAS Institute, Cary, NC, USA). A total of 16 MCL cases were identified (median age 65 years, range 41–75; 50% females), diagnosed between 2002 and 2022. Six (37%) patients presented with de novo MCL while the remaining 10 (63%) had antecedent mast cell disease (SSM 4, ASM 3, SM-AMN 2, CM 1); the 3 SM-AMN cases included 2 with chronic myelomonocytic leukemia (CMML) and one with MDS/MPN-unclassifiable. Four (25%) patients had biopsy-confirmed cutaneous involvement. Ten (63%) and 9 (56%) patients had palpable hepatomegaly and splenomegaly, respectively. Median (range) hemoglobin level was 9.7 g/dL (7.2–11.9), leukocytes 8.7 × 109/L (0.7–43.9), platelets 123 × 109/L (14–633), serum tryptase 614 ng/mL (69–1684), and serum albumin 3.7 g/dL (2.6–4.7). B- and C-findings were present in 15 (94%) and 12 (75%) patients, respectively. Seven (44%) and 5 (31%) patients had circulating MCs and blood eosinophil count >500/μL, respectively; only 2 (13%) patients had circulating MCs ≥10%. Nine of 13 (69%) and 3 of 14 (19%) evaluable patients harbored KITD816V and abnormal karyotype, respectively; information on alternative KIT mutations and treatment details was not available. Eight (50%) patients displayed a predominantly (>50% of total MCs) “immature” MC morphology (Figure 1), thus meeting ICC criteria for the diagnosis of MCL.2 MC morphology was “mature” in the remaining 8 (50%) patients, whose diagnosis of MCL was based simply on the presence of ≥20% MCs, as per WHO criteria, revised 4th and 5th edition.5, 8 Immature MC morphology clustered with CD2 (86% vs. 29%; p = .03) and CD25 expression (100% vs. 75%; p = .08) while mature MC cytology clustered with KIT D816V (86% vs. 50%; p = .15). Median (range) follow-up was 3.9 months (0.2–164); 4 patients were alive at 0.5, 1, 6 and 164 months (mature morphology); the latter patient was one of 3 patients who received AHSCT. Factors associated with significantly inferior overall survival (OS), on univariate analysis, were immature MC morphology (p < .01) and absence of KIT D816V (p = .02). There was borderline significance for peripheral blood eosinophil count ≥0.5 × 109/L (p = .052). On multivariable analysis, immature MC morphology (p = .03; HR 7.4, 95% CI 1.1–51.3) and absence of KIT D816V (p = .04; HR 6.8, 95% CI 1.0–49) were independently predictive of worse survival. Figure 2 depicts overall survival stratified by immature versus mature MC cytomorphology (median survival 0.7 vs. 23 months; p = .006; HR 10.1, 95% CI 2.0–52.8; Figure 2A), absence or presence of KIT mutation (median survival 0.7 vs. 22 months; p = .03; HR 7.4, 95% CI 1.3–42.0; Figure 2B), and combination of morphology and KIT D816V mutation status, that is, KIT D816V mutation-negative or immature morphology versus mutation-positive and mature morphology (median survival 0.9 vs. 33 months; p = .02, HR 13.1, 95% CI 1.6–109.5; Figure 2C). MCL is a relatively rare mast cell neoplasm representing <5% patients with systemic mastocytosis and is characterized by poor prognosis, across several studies.10-15 The most recently communicated study included 92 patients from the European Competence Network on Mastocytosis (ECNM) registry: 34% had an associated hematologic neoplasm, 14% met criteria for chronic MCL, and 4.5% displayed ≥10% circulating MCs11; KIT D816V was detected in 73% and other KIT mutations in 11%; abnormal karyotype was reported in 17% and high molecular risk mutations (SRSF2, ASXL1, RUNX1) in 44%. The particular ECNM study cited 4.2% progression rate to MCL from advanced SM and 0.2% from non-advanced SM.11 The most frequently used treatment agents in the ECNM study were midostaurin and cladribine and the authors suggested longer survival in patients who were exposed to midostaurin (median 2.3 vs. 1.1 years); among eight patients who received AHSCT, median post-transplant survival was 0.8 years.13 Among clinical trial MCL patients receiving either midostaurin16 or avapritinib,17 overall response rates of 50% and 69%, respectively, were reported. The major limitation for the majority of the studies described above was the lack of morphologic scrutiny to distinguish between “true” MCL with immature MC morphology versus SM with high but morphologically mature MC burden. We would argue that dependence solely on a 20% cutoff level for MC burden on BM aspirate smears, in order to diagnose MCL, might inadvertently include less aggressive variants, which otherwise would have been labeled as ISM, ASM, or SM-AMN. Hence, it is difficult to accurately interpret the results of current literature on the natural history and treatment response of patients with MCL. In the current study, we found immature MC morphology of the atypical MCs and absence of KIT D816V to be independently predictive of inferior OS in MCL patients and their combined consideration resulted in identifying a biologically distinct patient population with a uniformly poor clinical outcome (i.e., “true” MCL). Our observations are consistent with the 2022 ICC requirement for the presence of atypical immature MCs (i.e., promastocytes, metachromatic blast-like forms, and multinucleated or highly pleomorphic MC), for diagnosing MCL. Animesh Pardanani, Ayalew Tefferi, Aref Al-Kali, Mrinal Patnaik, William J. Hogan, Kebede Begna, Michelle A. Elliott, Jeanne M. Palmer, Nandita Khera, and Naseema Gangat participated in patient care. Attilio Orazi, Katalin Kelemen, Kaaren K. Reichard, and Dong Chen provided hematopathology expertise. Animesh Pardanani, Ayalew Tefferi, Kaaren K. Reichard, Dong Chen, and Attilio Orazi participated in concept and design of the study. Animesh Pardanani and Ayalew Tefferi wrote the paper. All authors reviewed and approved the manuscript. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.